Generally, the industry of semiconductor manufacturing involves highly complex techniques for fabricating integrating circuits using semiconductor materials that are layered and patterned onto a substrate, such as silicon. An integrated circuit is typically fabricated from a plurality of reticles. Generation of reticles and subsequent optical inspection of such reticles have become standard steps in the production of semiconductors. The fabrication of semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of semiconductor fabrication processes with multiple reticles to form various features and multiple levels of the semiconductor devices. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
If there are defects on the reticle or wafer, the resulting semiconductor devices may not function properly. Accordingly, various inspection system features and techniques have been employed to improve defect detection sensitivity for both reticles and wafers. One factor that affects accurate defect detection is the particular technique employed to control focus of the inspection tool. Such focus control can be referred to as autofocus.